Production of wrought titanium base alloys and resulting product



PRODUCTION OF WROUGHT TITANIUM BASE ALLOYS AND RESULTING PRODUCT Milton B. Vordahl, Beaver, Pa., assignor, by mesne assignments, to Crucible Steel Company of America, Borough of Flemington, N.J., a corporation of New Jersey No Drawing. Filed Apr. 21, 1954, Ser. No. 424,569

Claims. (Cl. 75-1755) This invention pertains to strong and ductile wrought titanium base alloys, characterized by a microstructure consisting of a coherent admixture of small bodies of alpha titanium and beta titanium, and to methods of producing the same, and to wrought articles thereof.

The invention pertains more particularly to wrought titanium base alloys of the character aforesaid, containing about 2.5 to atomic percent in total, of one or more elements selected from the group consisting of manganese, chromium, molybdenum and iron. The binary alloys of the invention will thus, of course, contain about 2.5 to 15 atomic percent of either manganese, chromium, molybdenum or iron. The corresponding weight percents are approximately 316% each for iron and manganese, about 3-l5% for chromium, and about 525% for molybdenum. For the ternary and higher order alloys, the total content of these elements shall be, as above stated, about 2.5 to 15 atomic percent, with lower eifective limits for the individual elements of about 1% by weight each for molybdenum, manganese and iron and about 3% by weight for chromium. The alloys of the invention are strengthened without undue embrittlement by additions of up to about 0.5% carbon, up to about 0.3% of oxygen, and up to about 0.1% nitrogen.

These alloys may be produced by are melting in a cold-mold furnace, in an inert atmosphere such as argon, or by equivalent procedures. The titanium employed may be either the high purity iodide titanium or the commercial purity product obtained by the magnesium reduction of titanium tetrachloride, or an equally pure material as obtained by other procedures.

This application is a continuation-in-part of my copending applications Serial No. 132,327, filed December 10, 1949, now abandoned, and Serial No. 229,143, filed May 31, 1951, and of the joint application of Walter L. Finlay and myself Serial No. 79,556 filed March 4, 1949, and now abandoned.

The low temperature or alpha phase of substantially pure titanium, which is of close-packed hexagonal structure, transforms at a temperature of about 885 C., to the high temperature or beta phase, which is of bodycentered cubic structure. The presence of such alloying elements as carbon, oxygen, nitrogen and aluminum tends to raise the beta transformation temperature, and establish a relatively narrow zone or field of mixed alphabeta structure. The presence of certain other alloying metals, prominent among which are iron, chromium,

ice

manganese and molybdenum, has a quite difierent effect upon transformation. Increasing amounts of these alloying metals stabilize the beta phase at progressively lower temperatures, and establish a mixed alpha-beta field of substantial scope.

The present invention comprises, in one of its aspects, the discovery of a method for imparting to titanium base alloys of the type aforesaid an interleaved dispersion or distribution of the alpha and beta phases, and of a unique microstructure resulting therefrom. An essential step in the method is the plastic deformation of the alloy at a temperature within the two-phase or mixed alphabeta field. Another essential step is the selection of a titanium base alloy composition possessing two or more ductile phases stable over a suitable temperature range. Such plastic deformation of such a structure differs widely from the generally accepted practice of deforming to a desired size and configuration in a single phase field, and subsequently heat treating to secure a desired set of properties. In the practice of applicants method, the essentials of the desired structure are. established by plastic deformation, and while subsequent heat treatment is permissible and may be further beneficial, it must be so controlled as not to destroy the essential structure set up by plastic deformation.

The alloys amenable to the present invention are of the mixed-phase structure. They are characterized by the presence of two phases of unequal strength, both of which possess some ductility but which deform by basically different crystallographic mechanisms. Thus, a stress concentration in one phase tends to difiuse, rather than to propagate, when it encounters the other phase. The titanium base alloys particularly disclosed by this specification possess all of the unusual set of qualifications: their alpha phase is relatively weak but quite ductile; their beta phase is relatively strong while nevertheless maintaining some ductility; and alpha, being a hexagonal close-packed structure, deforms in a basically different manner than does beta, which has a bodycentered cubic structure.

The present invention is particularly directed to sheet, wire and rod products in which service stresses are applied substantially parallel to the rolling or drawing direction and in which, therefore, the optimum distribution of the weaker phase is in thin fibrils or platelets whose longitudinal axes are parallel to the rolling or drawing direction. The presence of the weaker phase in thin fibrils or platelets disposed parallel to the working direction, presents a minimum area of weaker phase normal to the applied stress and so constitutes the strongest distribution, Conventional processing procedures such as working in the all-beta region followed by cooling at any convenient rate produce a distribution of the weaker phase which presents an appreciably greater area of weaker phase normal to the applied stress than that achieved by the novel processing disclosed in this invention and so is weaker. A further important advantage of the optimum distribution is that a stress concentration arising in the stronger phase (which is making the major contribution to supporting the applied stress) tends, in starting to propagate across the fibrils and platelets of the weaker phase, to be diliused in a manner analogous to the reaction to stress of a cable in contrast to that of a rod.

Each of the phases in the optimum distribution disclosed by this invention may be either continuous or discontinuous in the worked direction. The stronger phase should predominate in order to give higher strength and thus tends to be continuous in the worked and, therefore, the loadbearing direction. Even if it is not continuous, however, its fibrils and platelets are coherent with their contiguous material and so, like the fibrils in a textile thread, act as if they were continuous. It is an essential feature of this invention that, normal to the direction of Working, neither phase is continuous.

The history of the alloy prior to plastic deformation in the two-phase field is of little, if any, significance. The effect of plastic deformation in the two-phase field is to increase ductility at a given strength level, and this effect is quite independent of prior history. Whether the specimen has been hardened by quenching from a beta field or lower temperature, or has been fully annealed by slow cooling from a beta field temperature, or given any other treatment, extensive working in the two-phase field still develops its optimum properties, viz., the highest ductility at a given strength level. For example, the quenching from the all-beta field of a binary alloy of 4% manganese, balance substantially all commercial purity titanium, results in a structure characterized by extended martensite-like plates of alpha and irregular bodies of retained beta. The quenching from the all-beta field of a similar alloy containing about 7% manganese produces unstable beta grains. On subsequent elevated temperature aging or service there is a tendency to the growth of continuous bodies of alpha at the grain boundaries. Both structures are predisposed to fracture along the extended region of relatively weak alpha. If either alloy is slow cooled it becomes dead soft and is, for most uses, in its least serviceable condition. Extensive two-phase working of either of these alloys from either the quenched or the annealed condition develops properties identical with those developed by working the same alloy from the other condition. 7

Extensive plastic deformation such as can in rolling sheet or red or in drawing wire at a temperature within the two-phase field has the effect not only of breaking up and dispersing the continuous masses, particularly the relatively weak alpha, and reducing such continuous masses to much, smaller, substantially discontinuous and discrete particles, but also of producing the interleaved structureof fibrils and platelets of alpha and beta which is disclosed by this specification as being optimum. If one phase predominates, it tends to form a matrix in which island platelets or fibrils of the second.

phase are embedded.

'For example in a sheet of an alloy of commercial purity titanium with 4.1% manganese after a 90% area reduction by rolling at a temperature of 650 C. and stabilized for one hour at a temperature of 550? C., the dispersion and discontinuity of the phases, particularly the alpha phase, are substantially optimum for sheet, a good balance being struck between properties transverse and parallel to the working direction. The platelets are arranged parallel to the sheet surfaces. In a wire, the fibrils would be arranged parallel to the axis, and the structure is comparable with that of a cable as compared with a solid rod. The individual fibers of the stronger but more brittle phase can, to some extent, yield separately and thus distribute the load throughout the whole, while'the effect of stress-raising voids, lattice imperfections, and the like, tends to be nullified at the first junction with the weaker and more ductile phase.

' A typical specimen of the 4.1% manganese alloy has a yield strength in excess of 120,000 p.s.i., an ultimate strength above 145,000 p.s.i., an elongation in A2" of 16%, and a bend ductility of 3.3 T. 1 This ratio of duictility-to-strength is definitely superior to any obtainbe obtained 4 able from this alloy without extensive plastic deformation at a temperature within the two-phase field. The same is true of the numerous other alloys which have been found amenable to the processing of this invention. An alloy of 7% manganese, balance substantially all commercial purity titanium, after 75% area reduction by rolling at a temperature of 650 C., shows a structure substantially similar to that of the 4.1 manganese alloy. Prior to such warm working and after furnace cooling from 750 C., it showed a heterogeneous mass of various ly oriented extended bodies of the two phases. After 75 Warm rolling and stabilization for one hour at 550 C., alloys of commercial titanium with about 8% manganese show a yield strength of about 145,000 p.s.i., an ultimate strength over 157,000 p.s.i., an elongation in /2" of about 16%, and an average bend ductility of about 3 T.

While the broad invention contemplates plastic deformation at any temperature within the two-phase field at which the strength of the alloy permits such deformaion, the working temperature should not too closely approach the boundary temperature between the mixed alpha-beta field and the beta field. The heating of the alloy prior to Working, and possibly local heating induced by working itself promote transformations toward equilibrium at the particular temperature. If the working temperature is too close to the all-beta temperature, the tendency to beta formation is .so great that a substantial part of the benefits of the invention may be lost. In general, the working temperature should not approach the all-beta temperature within about 50 to 100; The preferred temperature range for plastic deformation is from about 400 C. to about 50-100 C. below the beta transus temperature.

The beta transus temperature for any particular alloy in accordance with the invention may be determined by quenching specimens from progressively higher temperatures, until a completely martensitic or retained beta micro-structure is obtained. 7

Increasing amounts of plastic deformation within the two-phase field efi'ect increasingly complete dispersion and interleaving of the phases, and proportionately increase ductility at a given strength level. Generally stated, strength is a function of composition and conditions of stabilization subsequent to two-phase temperature working. For an alloy of given composition, it has been found possible, by appropriate stabilization, to maintain a substantially constant Strength-level throughout varying amounts of plastic deformation. Any material variations in strength being thus eliminatcd, the progressive increase in ductility with increasing two-phase temperature deformation has been established, without complication by other variables. 'The experimental work leading to the present invention was exhaustive, comprising different alloys, different treatments prior to two-phase working, different working temperatures, and varying mounts of two-phase temperature deformation.

As an example of the work, the following series of tests is described. Three alloys were selected which contained, respectively, 3.5%, 4.1% and 4.7% manganese, balance substantially all commercial purity titanium. An ingot of each composition was forged at 870 C. to a /2" to slab, each slab was cleaned and rolled at about 815 C., to sheet of four diiferent thicknesses so chosen that the varying amounts of .warm rolling reduction (to be described) resultedin specimens of the same thickness, i.e., all finished specimens received the same total reduction from the ingot. Each of the twelve sheets'was subdivided into three parts, and the three parts respectively of each sheet were conditioned The measurement of bend ductility is not standardized. The present applicant and his associates measure this prop erty as the radius over which the'specimen can be bent to an angle of 75 without cracking,.the radius being expressed as a multiple of specimen thickness.

for two-phase temperature rolling by three difierent treatments, as follows:

Condition Q Heated in air one hour at 815 0., water quenched;

Condition Q-T Heated in air one hour at 815 C., Water quenched, and tempered sixteen hours at 550 C.;

Condition F-C Heated in air one hour at 815 C.,

furnace cooled.

All specimens were then rolled to a uniform thickness at a two-phase field temperature, three different temperatures being used. The thickest specimens of each set were reduced 90%, the next thinner 75%, the next thinner 60%, and the thinnest 30%, the final rolled thickness of all being the same. All specimens were then stabilized for one hour at 550 C. Strength and ductility were then measured, and found to be as follows:

Composition: 3.5% manganese-balance, commercial titanium. Rolling temperature: 650 C. Stabilized: 1 hour at 550 0., after rolling.

Bend Duc- Pre-Rolling Reduc- Yield Ultimate Elongatility Condition tion, (1,0005 (1,0005 tion, (Least percent p.s.i.) p.s.i.) percent Favorable Direction) Composition: 4.7% manganese-balance, commercial tltanium. Rolling temperature: 700 C. Stabilized: 1 hour at 550 0., after rolling.

Bend Duc- Pre-Rolling Reduc- Yield Ultimate Elongatility Condition tion, (1,000s (1,000s tion, (Least percent p.s.i. p.s.i.) percent Favorable Direction) Composition: 4.1% manganese-balance, commercial titanium. Rolling temperature: 565 C. Stabilized: 1 hour at 550 0., after rolling.

6 In the foregoing tables, each of the reported values is the average of four diiferent tests. The temperature of 815 C., to which all of the alloys were heated prior ,to the two-phase rolling, if not within the all-beta field lies so close to the all-beta field as to destroy the effect of previous working. The tabulated results definitely show for each alloy rolled from each of the three prerolling conditions, first, a negligible variation in yield and ultimate strength with varying amounts of twophase temperature rolling; and, second, a progressive increase in ductility for increasing amounts of such rolling. 'Ilhese efiects are the same at any of the three rolling temperatures. Other variables being eliminated, increasing amounts of plastic deformation in the twophase field effect a progressive increase in ductility.

Stabilization, as above described, is desirable under most conditions, and can be performed at temperatures up to about 550 C., without materially altering the microstructure.

While the optimum combination of strength and ductility is secured by extensive plastic deformation in the two-phase field, fabrication problems frequently render it impossible to roll sheet to the strength level desired in the fabricated product. Bending, forming, drilling, and like operations, cannot be satisfactorily performed at the desired high strength. It has been found that the alloys processed according to the present invention can be annealed to the low strength level necessary for fabrication, and, after fabrication, quenched to the desired high strength, and still retain the essentials of the dispersed phase structure. The alpha phase is spheroidized, but remains as discrete bodies rather than continuous masses.

An alloy containing 6.8% manganese, balance commercial purity titanium, which, after extensive plastic deformation in the two-phase field, had been heated for 1 hour at 700 C., and water quenched, was found to consist of a beta matrix containing very numerous relatively small and substantially discrete globules of alpha. This specimen, after stabilization for 1 hour at 500 0., showed a yield strength of about 140,000 p.s.i., an ultimate strength of about 170,000 p.s.i., an elongation in V2" of about 12%, and a bend ductility of about 3 T.

While in the foregoing description the binary alloys of titanium and manganese have been used for the purpose of illustration, it has been found as the result of very extensive work that the novel processing is equally applicable to any alloys comprising two ductile phases, both of which are relatively stable at normal temperatures. Prominent among these are the binary alloys of titanium with molybdenum, chromium and iron, as well as manganese, and the ternary and higher alloys of titanium with the metals of this group, and with aluminum up to about 8%. While in the binary titanium-aluminum alloys the beta phase is not stable at normal temperatures, it is stabilized by the addition of one or more of the metals, manganese, molybdenum, chromium and iron. The weight percentages of the different metals of the group, manganese, molybdenum, chromium and iron, which, when added to titanium produce a two-phase structure, vary with the particular metalthe maximum for manganese, for example, being about 16%, but the atomic percentages fall within the range of about 2.5 to 15 atomic percent. Below about 2.5 atomic percent the beta phase is either absent altogether or present only in a negligible amount; while above about 15 atomic percent the alloys show an all-beta structure on quenching. Typical alloys and their properties as worked in the two- 7 g 7 phase field, both *m'th and without subsequent stabilizalma-areas follows: .7 V I s minimumtensile strength of at least 137,000 p.s.i. and

elongation of at least 2%.

*Stabilizedat 700 0. Cold worked. 7

In my copending application Serial No. 132,328,filed December 10, 1949, now US. Patent No. 2,704,251, dated March 15, 1955, l have shown that binary alloys of titanium and manganese containing about 6 to 12% manganese are characterized by excellent strength and ductility. In thepresent application I have shown that thexbinary titanium-manganese alloys containing from about 3% to less than 6% manganese, when produced in wrought form by plastic deformation in the alpha-beta field likewise possess high strength and ductility.

The alloys of the invention having tensile elongations as low as about 2% are useful in massive form, for example, forg'ings, and are useful in the form of rolled sheet or drawn wire with minimum bend ductilities as high as 20 T.

By way of comparison with the above results, the mechanical properties of the unalloyed titanium base metal as hotrolled below the betaitransus temperature, vary somewhat with the amount of contaminants present, butin general fall within the following'limits: 0.2% oflset yield strength 65,00080,000 .p.s.i; ultimate strength 75,00090,000 p.s.i., tensile elongation 20-25%.

What is claimed is:

1. An alloy-consisting essentially of from 1.0% to 5;0% manganese, from 1.0% to 2.0% iron, with the minimum iron plus manganese content being about 2.5 atomic percent, and the balance titanium, characterized by .a-tensile strength of at least about 25% in excess of the tensile strength of unalloyed titanium, measured in the as-hotrolled (at about 1200" 'F.) condition.

2. An alloy consisting essentially of from about 3% to about 7.5% chromium, from about 0.2% to about 0.5% carbon, and the balance titanium, said alloy being characterized by having a minimum tensile strength of about 128,000 p.s.i. and a minimnmelongation of about :5.0% in the as-hot-rolled (at about 1250"- F.) condition.

3. An alloy consisting essentially of from about 2.5% to about 5 %,molybdenum, about 0.1% nitrogen, and the balance substantially'titaniurn, said alloy being characterized by a minimum tensile strength of at least 130,000 p.s.i. and a minimum elongation of at least 2.5%

4. An alloy consisting essentially of from about 1% .to about 5% manganese, about 0.1% nitrogen, from Tensile Properties -Oomposition Percent As Warm Rolled in the Alpha-Beta Field Asstablhzed at 600 0.

(Balance Titanium) 4 'Elon atlon Bend .2 Yield Ultimate Elongation Bend I 7 Ultimate 1, Radius 1 r in W. Radius 153, 000 173, 000 12 '6. 5 138, 000 149,000 18 3.3 152, 000 175,000 13 6. 5 144, 000 160, 000 22 4. 9 159, 000 84, 000 2 4. 1 132; 000 146, 000' 12 2. 9 170, 000 187,000 6 6. 2 155, 000 160, 000 12 4. 134, 000 154, 000 7 4. 6 132,000 143, 000 14 2. 7 141, 000 160, 000 7. 0 133, 000 142, 000 11 2. 7 117,000 146, 000 12 2. 7 98, 000 125, 000 19 .1. 7 138, 000 164,000 8 8:8 119, 000 141, 000 ,8 6.5 135, 000 159, 000 16 7. 5 122, 000 145, 000 20 6.2 140 000 170, 8 6. 1 135,000 159, 000 5. 8 271 154, 000 14 4. s 117, 000 ,000 21 2. o 136, 000 164, 000 8 4. 9 127, 000 139, 000 19 2. 7 155, 000 175, 000 10' 6. 5 138, 000 150, 000 6. 6 155, 000 113,000 g 3 3. :68 6Mn4.30r2.9Mo-0.42G 153, 000 171, 000 3,3MnlFe-4A10.2C0.02N 127, 000 158, 000 10 5- 7 118, 000 138, 000 15 2. 2 8.2Mnl.0Fe 163,000 169, 500 11. 7 9.4Mn-5.2M0 128, 100 146, 100 7 9.1Mn5Cr5.1h/Io- 135, 700 158, 400 10 5.1M0'10.1M115.2Cr- 138,900 144, 600 23 9.3Mn-5.1Gr4.7M0 197, 000 215, 800 2 5. An alloy consisting essentially of from about/2.0% to about 5.0% molybdenum, about 0.1% nitrogen, from about 0.25% to about 0.5% carbon, and the balance substantially titanium, said alloy being characterized by a minimum tensile strength of at least 143,000 p.s.i. and minimum elongation of at least 2%.

6. Analloyconsisting'essentially of from about 1.5% to about 7.5% -mauganese, about 0.1% nitrogen, and the balance substantially all titanium, said alloy being characterized by a minimum tensile strength of at least 134,400

p.s.i. and a minimum elongation of at least 2.0%. p

7.. A titanium-base alloy consisting essentially of about: 2.5 to 15 atomic percent of atleast two beta promoting elements selected from the group consisting of manganese, chromium, molybdenum and iron, chromium when present with molybdenum .alone, being not under 3% by weight, up to 0.5% carbon, up to 0.3% oxygen, up

about 0.25% to about 0.5% carbon, and the balance substantially titanium, said alloy being characterized by a 'to 0.1% nitrogen, up to 8% aluminum, balancesubstantially titanium, characterized as plastically deformed in the alpha-beta temperature'field, by tensile strength of at least 130,000 p.s.i. and a tensile elongation of at least 2%.

8. A titanium-base alloy consisting essentially of about: 2.5 to 15 atomic percent of at least three elements selected from the group consisting of manganese, chromium, molybdenum and iron, carbon up to 0.5%, oxygen up to 0L3 nitrogen up to 0.1%, up to 8% aluminum, balance substantially titanium, characterized as plastically deformed in the alpha-beta temperature field, by a tensile strength of at least 130,000 p.s.i. and a tensile elongation of at least 2%.

-9. A titanium-basealloy consisting essentially of about:

2.5 to 15 atomic percent of at least two elements selected from the group consisting of manganese, chromium, molybdenum and iron, 0.5 to 8% aluminum, up to 0.5% carbon, up to 0.3% oxygen, up to 0.1 nitrogen, balance substantially titanium, characterized by high strength and ductility.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS 2,206,395 Gertler July 2, 1940 2,287,883 Kroll June 30, 1942 2,412,447 Donachie Dec. 10, 1946 2,588,007 Iafiee Mar. 4, 1952 2,640,773 Pitler June 2, 1952 2,857,269 Vordahl Oct. 21, 1958 OTHER REFERENCES Kroll: Zeitschrift fur Metallkunde, vol. 29 (1937), pages 190, 191.

Transactions of A.I.M.M.E., vol. 166 (1946), pages 390-396.

Titanium Project, Navy Contract No. Noa(s) 8698 (Mallory), Report No. 10; dated Feb. 16, 1948 (prior knowledged), pages 1, 3-9, 11-14.

Summary Report, Part III covering period May 18, 1948 to July 30, 1949 on Preparation and Evaluation of Titanium Alloys as reported to Wright-Patterson Air Force Base, Dayton, Ohio by Battelle Memorial Institute, Columbus, Ohio. Pages-Letter of Transmittal (2 pages) and 324, 325 and 328a.

"Transactions of American Society for Metals, vol. 41 (1949), pages 985-992.

Metal Progress, October 1954, page 164. 

1. AN ALLOY CONSISTING ESSENTIALLY FORM 1.0% TO 5.0% MANGANESE, FROM 1.0% TO 2.0% IRON, WITH THE MINIMUM IRON PLUS MANGANESE CONTENT BEING ABOUT 2.5 ATOMIC PERCENT, AND THE BALANCE TITANIUM, CHARACTERIZED BY A TENSILE STRENGTH OF AT LEAST ABOUT 25% IN EXCESS OF THE TENSILE STRENGTH OF UNALLOYED TITANIUM, MEASURED IN THE AS-HOTROLLED (AT ABOUT 1200*F.) CONDITION. 